1. Field of the Invention
This invention relates to a soil-displacing screw auger which contains an auger tube; a double displacement body equipped with a hollow shaft fixed to the auger tube and containing an uppers part which constitutes a closed screw the radius of which increases spirally towards its lower extremity and a lower part which constitutes a closed screw the radius of which decreases towards its lower extremity, preferably with the same pitch as the upper part; a lower tube the diameter of which is smaller than twice the largest radius of the displacement body and which is situated between the lower part of the displacement body and the lower extremity of the auger; a screw blade welded onto the outer casing of the lower tube, which rotates in the same sense as the lower part of the displacement body; and an auger tip connected to the lower extremity of the lower tube.
This type of auger is used to make concrete piles in the ground, which are used as foundations for buildings if the top soil layers are compressible and normal foundations would lead to an inadmissible settlement.
2. Description of the Related Art
The soil is displaced by the double displacement body, so that pressure release in the soil is avoided during the creation of the pile and a smaller quantity of soil is removed from the borehole.
Once the auger has reached a sufficient depth, concrete is poured into the auger tube and the auger is screwed out of the soil. When screwing out, the auger is rotated in the same sense of rotation as during the screwing in. The tip is a lost tip and stays in the ground.
Such an auger has been described in EP-B-0.228.138 in the name of the applicant. In this auger, the lower tube has the same diameter as the auger tube and in practice, this lower tube is a part of the auger tube which extends both underneath and above the displacement body.
With this auger, only a limited depth in good soil, only one meter, can be reached.
The reason for this is that, when screwing into good soil, the soil will accumulate between the turns of the screw blade and very soon, no more soil will be transported to the surface. The entire underside of the auger then displaces the soil, causing the resistance to the screwing into good soil to become too large very soon.
Given their relatively small diameter, piles must have a very large permissible load and penetrate deeply into heavy ground, which is not possible with this type of auger.